On Saturday, Chavez said he wanted Maduro to replace him if "something were to happen that would incapacitate me."

If the president dies, Venezuela's Constitution specifies that the vice president assumes the presidency until new elections can be held.

Chavez called for voters to take things a step further.

"My firm opinion, as clear as the full moon -- irrevocable, absolute, total -- is ... that you elect Nicolas Maduro as president," Chavez said, waving a copy of the Venezuelan Constitution as he spoke. "I ask this of you from my heart. He is one of the young leaders with the greatest ability to continue, if I cannot."

It was the first time Chavez had spoken publicly about the possibility of a successor -- a shocking subject from a man who looms larger than life in Venezuela and in Latin American politics.

"This is huge. He could have said something indirectly. He could have said something like, 'We'll have to see. Let's talk about it when the time comes,'" said Javier Corrales, a professor of political science at Amherst College in Massachusetts. "He switched from being very evasive to very articulate. That must have been the result of a major change in health for the worse."

Last week, Chavez returned from Cuba after receiving medical treatment. He said doctors detected malignant cells and that he expects to undergo surgery in the coming days.

Chavez, who had surgery in 2011 to remove a cancerous tumor, has undergone further operations and radiation therapy in Cuba since then. He declared himself cancer-free in July.

Health rumors dogged Chavez on the campaign trail this year, but didn't stop him from winning re-election in October.

The president has repeatedly spoken publicly about his cancer battle, but has never specified what type he has.

The government has released few specifics, fueling widespread speculation about his health and political future, and sparking criticism from political opponents.

"This has not been handled with the transparency and the truth that our people deserve," opposition Rep. Julio Borges said Sunday.

But Diosdado Cabello, a close Chavez ally and the president of the National Assembly, countered that claim.

"By God, since the first day he has spoken clearly. These men are trying to have a party with the president's illness," he said. "It's going to turn out well. He continues being the head of state and the head of the government. He continues being the president of the republic, under any circumstance."

State-run VTV Sunday showed throngs of supporters cheering and praying in support of Chavez.

An opposition coalition leader told CNN affiliate Globovision that the opposition would be prepared for a new presidential vote.